The present invention relates to a hydraulic control system for a transmission of an automotive vehicle.
Automatic transmissions generally used in automotive vehicles comprise a torque converter and a transmission gearset which automatically shifts itself to a desired gear range by selectively actuating a plurality of frictional coupling elements.
One known type of automatic transmission is a steplessly variable transmission having a belt-pulley transmission mechanism which comprises a pair of, or primary and secondary, pulleys variable in pitch or effective diameter, and an endless belt rotationally coupling the primary and secondary pulleys together. The primary pulley is connected, either directly or indirectly, to an engine output, and the secondary pulley is connected to a drive line. The primary and secondary pulleys complementarily change in effective diameter to steplessly vary the ratio of transmission of the belt-pulley transmission mechanism. Such a steplessly variable transmission is known from, for example, Japanese Patent Publication No.62-4958.
A hydraulic control circuit is most commonly used to selectively actuate the frictional coupling elements of the automatic transmission, change the primary and secondary pulleys in effective diameter, and change tension of the endless belt of the steplessly variable transmission. The hydraulic control circuit controls a regulating valve to adjust hydraulic pressure from a hydraulic pump to a certain line pressure. The adjusted line pressure is delivered to hydraulic actuators for the frictional coupling elements of the automatic transmission, or to hydraulic actuators for changing the effective diameter of the pulleys of the steplessly variable transmission, through various valves. The line pressure is increasingly or decreasingly changed so as to control the capacity of torque transmission of the frictional coupling elements, or of the pulleys, correspondingly to the output of engine and the ratio of transmission.
In recent years, it has become popular to use a duty solenoid valve, which alternately opens and closes at repeated short time intervals, for regulating a line pressure of the regulating valve. The duty solenoid valve is driven or actuated at a duty rate, i.e., the ratio of opening time to one cycle of open-to-close time, so as to regulate a pilot pressure delivered to the regulating valve, thereby correspondingly creating a desired line pressure.
Because of the periodic opening and closing, the duty solenoid valve causes a pilot pressure applied to the regulating valve, and hence a regulated line pressure delivered from the regulator valve, to pulsate. Pressure pulsations have adverse effects on controlling the transmission of torque and the ratio of transmission of the frictional coupling elements of the automatic transmission or the belt-pulley transmission mechanism of the steplessly variable transmission.
To avoid adverse effects of pressure pulsations, it is well known to operate, or open and close, the duty solenoid valve at short intervals. That is, the duty solenoid valve is operated by an operating signal provided at a high frequency. However, this results in a deterioration of durability, or in extreme cases, a decline in reliability, of the duty solenoid valve.